(a) Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine or printer, and, more particularly, to a method of adjusting a spot diameter with a scanning-type optical device.
(b) Description of the Related Art
Referring to FIGS. 11 and 12, the related art will be described below.
FIG. 11 illustrates the entire structure of an image forming apparatus. A photosensitive drum 21 that is an example of an image bearing member has a photosensitive layer applied onto a conductive body, and forms an electrostatic latent image with laser beams emitted from a scanning-type optical device 23. Reference numeral 22 denotes an image reading unit that reads image information from an original placed onto an original base plate. Reference numeral 23 denotes the scanning-type optical device that irradiates the photosensitive drum 21 with the laser beams based on the image information transmitted from the image reading unit 22 or a personal computer or the like. Reference numeral 24 denotes a charger that charges the photosensitive drum 21. Reference numeral 25 denotes a developing unit that forms a toner image with triboelectrically charged toner, based on the electrostatic latent image formed on the photosensitive drum 21. Reference numeral 26 denotes a sheet feeding cassette that stores sheets that serve as recording materials. Reference numeral 27 denotes a fixing unit that fixes the toner image transferred onto a sheet by virtue of heat. Reference numeral 28 denotes a post processing unit on which image-fixed sheets are collectively placed. Reference numeral 29 denotes a cleaner that removes the toner remaining on the photosensitive drum 21 after the image transfer.
The scanning-type optical device 23 emits laser beams based on image information onto the photosensitive drum 21, so as to form an electrostatic latent image onto the photosensitive drum 21 charged by the charger 24. Toner triboelectrically charged in the developing unit 25 is then attached onto the electrostatic latent image, thereby forming a toner image on the photosensitive drum 21. The toner image is transferred onto an intermediate transfer belt from the photosensitive drum 21. The toner image is further transferred onto a sheet conveyed from the sheet feeding cassette 26 located at the lower part of the apparatus, so that an image is formed on the sheet. The toner of the image transferred onto the sheet is fixed by the fixing unit 27, and the sheet is placed onto the sheet output tray of the post processing unit 28.
Referring now to FIG. 12, the scanning-type optical device 23 is described. Reference numeral 30 denotes a laser beam source that emits beams based on image information. Reference numeral 31 denotes a collimating lens that turns each laser beam emitted from the laser beam source 30 into a collimated light beam. Reference numeral 32 denotes a cylindrical lens that concentrates each laser beam to a belt-like form. Reference numeral 33 denotes a rotary polygon mirror that deflectively scans with a laser beam. Reference numeral 34 denotes a first imaging lens that concentrates a laser beam onto the photosensitive drum 21. Reference numeral 35 denotes a synchronization sensor that senses synchronization in the main scanning direction. Reference numeral 36 denotes an imaging lens that concentrates a laser beam that enters the synchronization sensor 35. Reference numeral 37 denotes an optical housing that houses the above described components. The opening of the optical housing 37 is covered with a lid.
In the image forming apparatus equipped with the scanning-type optical device 23, the temperature inside the image forming apparatus rises due to the heat generated from the fixing unit 27 and each motor or the like, and the spot diameter on the photosensitive drum 21 might become greater than a predetermined value. This is because there are changes in the refractive indexes of the lenses provided in the scanning-type optical device 23, due to the heat.
Also in the laser beam source, there are cases where the diameters of emitted laser beams differ from one another due to a temperature rise caused by the laser beam emission.
To counter this problem, glass lenses that exhibit smaller refractive index changes than the changes in the refractive indexes of the above described lenses are employed to restrict the variation of the spot diameters on the photosensitive drum 21. Japanese Unexamined Patent Publication No. 2002-006211 discloses a structure that sets the refractive indexes of lenses so as to cancel a focus deviation caused due to a temperature rise. However, any of the conventional methods cannot sufficiently solve the problem of spot diameter variations due to a temperature rise in the laser beam source.
To further counter this problem, Japanese Unexamined Patent Publication No. 10-142546 discloses a structure that adjusts a focus deviation by inserting a focus lens between an fθ lens and a photosensitive drum. Japanese Unexamined Patent Publication No. 09-105876 discloses a structure that measures each spot diameter with a CCD line sensor, and adjusts the spot diameter by moving the cylindrical lens. Japanese Unexamined Patent Publication No. 07-261103 discloses a structure that measures the magnification in each scanning operation with a sensor provided at either end outside the image region, and moves a mirror so as to reduce the magnification error.
In any of the above conventional scanning-type optical devices, a spot diameter is adjusted by moving a lens or a mirror along the light path. Therefore, it is necessary to secure a sufficient width for the movement and to employ a motor or the like. As a result, the apparatus becomes large in size.